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726

Thomsons Turbine.

L L lying in the groove M M ; and as the ring Q is revolved, by
hand or governor, through gear N, the shutters are completely
raised or lowered, according to direction of rotation.

In Fig. 713 the actual path of the water is shewn in a Jonval
turbine at A, and in a Girard turbine at B, a b being free path and
velocity due to guide blades, and b c the wheel velocity; a c is the
relative velocity, and shews actual path in general direction.
Making c d = 'b c, ad will be the line of wheel vane causing
curved water path a <:, the horizontal ordinates of curvature on a d
and a c being equal


	
	
	
	
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Fig. 714 is a diagram shewing comparative efficiencies under
varying openings. Although the Girard is usually less efficient
than pressure turbines with full sluice, its efficiency is unimpaired
by fractional opening,

Thomson's Turbine, Fig. 715.—Here the supply water A enters
the rim of the wheel B, and escapes axially into c the tail race, so
the machine is called an inward-flow turbine. Its energy is
largely due to pressure, the outlet being either drowned or
connected with a suction pipe. Referring to the plan, the guide
blades D D are pivoted at E E, and can be moved in or out by the
levers and links F F. Then the vertical shafts at F F are all
connected, and rotated, through worm gear, by the hand wheel G;
thus more or less water mcty be admitted to the wheel. Although
the gear is complicated, its action is very perfect, the supply being
regulated without materially affecting angle of blades or other
conditions, and a nearly maximum efficiency of 75% obtained for
-11! onenings. The wheel is shewn in detail at H.